1. Field of the Invention
The invention generally relates to a heating element for hot applications. More particularly, the invention relates to a heating element for heating and evaporating in controlled manner vaporizable and/or tobacco-containing substances in electronic cigarettes.
2. Description of Related Art
Electronic cigarettes, also referred to as e-cigarettes below, are increasingly used as an alternative to tobacco cigarettes. Typically, e-cigarettes have a mouthpiece and an evaporator unit that comprises a heating element.
The heating element heats a vaporizable liquid so that the latter can be inhaled by the user. This liquid may already contain nicotine. Alternatively, the liquid is free of nicotine. In this case, the aerosol that is being formed may then flow through a nicotine containing and nicotine releasing body.
For example, lance-shaped heating elements are known from the prior art. These heating elements are introduced into a specially designed piece of tobacco and thus brought into contact with the substances to be evaporated to heat them to temperatures ranging from 50° C. to 350° C. This causes formation of an aerosol. Such heating lances may consist of a heating wire without a carrier material. However, a drawback hereof is that because of the required mechanical stability of the heating element the dimensions of the heating element cannot be made arbitrarily small. Furthermore, such heating elements tend to become easily contaminated during use.
Therefore, as an alternative, heating lances are described in the prior art which have heating conductor structures that are applied on a carrier material. These heating lances have ceramic carrier materials, since in addition to high temperature stability the latter provide electrical insulation. For example, EP 2 469 969 describes heating lances with carrier materials based on ZrO2 ceramics.
A drawback when employing ceramic carrier materials, however, is not only their high manufacturing costs, but also their high surface roughness and porosity. Roughness and porosity have an adverse effect on the heating conductor structures applied thereon in the form of a conductive coating. For example, the rough surface adversely affects the adhesion of the conductive coating to the carrier material.
Furthermore, the known ceramic carrier materials exhibit high thermal conductivity. This is unfavorable for the use in a heating element, since the heat generated in the heated portion of the heating element cannot be released into the medium to be heated in controlled manner, rather heat dissipation through the ceramic will occur and the heat dissipated in this manner will therefore no longer be available for the evaporation or heating of the substances. Accordingly, more heating power has to be provided by the heating element, which not only adversely affects the energy consumption and therefore the battery or recharge time of the e-cigarette, for example, but may also lead to a temperature increase in the e-cigarette and thus may have an adverse effect on the service life of the heating element.
In an alternative configuration of an e-cigarette, the heating element can be arranged within the e-cigarette so as to be not directly introduced into the piece of tobacco or the substances to be evaporated, but rather so as to enclose the piece of tobacco or a reservoir with the substances to be evaporated in cylindrical manner. Such an arrangement is described in US 2005/0172976, for example. Such external heating elements offer the advantage that the substances or tobacco pieces to be evaporated can be exchanged more easily. Due to the desired small dimensions of the e-cigarettes, which are typically modeled on the dimensions of conventional tobacco cigarettes, very small diameters and thus bending radii are resulting with such an arrangement of the heating element. Since, moreover, the carrier material has to be an electrical insulator, only high-performance plastics such as, for example, polyimides or polyamides have so far been used as the carrier material.
In such arrangements, performance and service life of the heating element is limited by the rather low temperature resistance of the plastics. Moreover, leaching effects may be caused by the organic solvents used in the e-cigarette. On the one hand, this is disadvantageous with regard to the service life of the heating element. In addition, constituents of the carrier material might be dissolved in the organic solvent and inhaled by the user.